SQL> create table mi8 (v numeric (30, 4));
SQL> insert into mi8 values(12.345);
SQL> commit;
SQL> create index i8 on mi8(v);
SQL> select * from mi8 where v = 12.345; -- result is correct

                                        V 

=============================================
12.3450

SQL> select * from mi8 where v = '12.345'; -- result is wrong
SQL>